The present invention relates to a raster scan display such as that used in a cathode ray tube (CRT). In particular, it relates to a circuit capable of providing a smooth raster scan display of the type used in monitoring or signal display equipment.
Normal bit mapped pixel graphics are often not satisfactory for presentation of medical waveforms due to the jaggedness which occurs when drawing lines which are not parallel to the deflection axes of the CRT. The jaggedness may be reduced by employing a large number of scan lines (and hence a very high deflection frequency) and by using a very high "dot clock" frequency. As the resolution of a display is increased, the cost of memory to support the display quickly makes this approach impractical for moderate cost medical monitors.
With a resolution of 720 lines wide by 2048 pixels high jaggedness is both noticeable and objectionable. Jaggedness may be reduced by varying the brightness of the CRT beam in a manner that smooths out the steps between adjacent illuminated portions of scan lines on a vertical raster display. For example, in U.S. Pat. No. 4,212,009, entitled SMOOTHING A RASTER DISPLAY which issued to G. L. Adleman et al. on July 8, 1980, a linear brightness control technique which makes use of computer controlled positive and negative current sources which vary the voltage on a capacitor which is then used to modulate the width of an electron beam during a raster scan is disclosed. As described therein, the slope of the voltage which is generated on the capacitor is determined by the output of a computer controlled constant current source. Accordingly, the technique involved the use of a pair of digitally controlled current sources and a switch to generate the up and down linear voltage ramps on the capacitor to form diamond shaped beam patterns. Unfortunately, ramp length, i.e. the time required for the voltage to reach a particular value, is not a linear function of current. In particular, EQU i=C(dv/vt)
where i equals the current value, C equals the capacitor value and dv/vt represents the rate of change of voltage with respect to time. Accordingly, doubling the current leads to a ramp having half the length. Using the approach described by Adelman et al., accurate control of the ramp length (slope) becomes very difficult at low current levels. It is also very difficult to accurately clamp the top of the ramp with currents which may vary over more than two orders of magnitude.
Accordingly, a linear approach to beam modulation which does not rely upon a highly variable current source and which includes provision for a generalized diamond shape such as a trapezoidal shaped pattern as well as a diamond shaped pattern for added flexibility in smoothing out the display would be desirable. Also, a method and apparatus for smoothing the display for digital waveforms which is not overly expensive due to large memory costs is also desirable.